This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The 26S proteasome is a macromolecular machine existing in all eukaryotes and is responsible for ubiquitin/ATP dependent protein degradation in both cytosol and nucleus. It consists of two subcomplexes, the 20S core particle (CP) and the 19S regulatory complex (RP). The 20S CP is responsible for various catalytic activities, while the 19S RP is involved in several biochemical functions including recognition and unfolding of polyubiquitinated substrates, assisting in opening the gate of the 20S chamber and subsequently translocating unfolded substrates into the 20S for degradation. Selective degradation in cells is tightly controlled and plays an important role for regulating cell cycle progression, signal transduction and maintaining genome stability, etc. Disruption of normal proteolytic destruction pathways can lead to a wide range of human disease, including cancer and neurodegenerative disorders. Despite intensive research, the unknown still exceeds what are currently known on ubiquitin/proteasome dependent degradation. In order to understand the regulation of the 26S proteasome function, we are interested in studying dynamic structural changes in the 26S proteasome complex under different physiological conditions.